This invention concerns an aluminum foil electrolytic capacitor in which the anode foil bears both a layer of hydrous aluminum oxide and a barrier layer dielectric oxide and is in contact with an electrolyte containing up to 5 wt% depolarizer and sufficient soluble phosphate to restore the maximum anodization voltage to the desired level.
The role of added phosphate ion in the working electrolytes for aluminum electrolytic capacitors is not fully understood. It is known that phosphate ion effectively inhibits the hydration of aluminum. This is the basis for adding small amounts of phosphate ion, usually as ammonium dihydrogen phosphate, to working electrolytes. This helps to insure the continued passivation of the cathode foil and to protect it against hydration. The hydration reaction is accompanied by copious hydrogen gas evolution, and the effect of this gas evolution is a frequent cause of capacitor failure.
In order to take care of this gas evolution, it would be desirable to add an effective depolarizer to react with the hydrogen as it is generated and thus prevent its buildup and capacitor failure.
However, electrolytic capacitor problems increase in severity as the working voltage of the capacitor increases and as the physical size of the capacitor decreases. With higher capacitance and higher working voltages, there is an increasing probability of encountering defect and impurity sites in the anodic oxide which leads to higher leakage currents, which in turn means more evolution of hydrogen gas at the cathode. This gassing frequently leads to first the bulging of the capacitor case and eventually to the disruption of the assembly and failure of the capacitor.
When the amount of depolarizer added to the electrolyte is increased to up to 5% to counteract the additional hydrogen evolution noted above, particularly in high voltage capacitors (200 V and higher), the higher concentration unfortunately decreases the voltage capabilities of the electrolyte to the point where the electrolyte is no longer suitable for such high voltage operation.
Not only has the severity of service requirements for electrolytic capacitors been increasing with regard to higher working voltages, higher capacitances, and smaller physical sizes, but also the temperature requirements have increased. Today, capacitors are designed to operate at 125.degree. C. and even 150.degree. C. rather than the usual 65.degree. C. or 85.degree. C. of the past. At these higher temperatures, the prior levels of phosphate ion concentration are proving to be inadequate. Thus, it would be desirable to increase the phosphate concentration particularly since there is growing evidence indicating its presence also helps to stabilize the oxide layer on the anode. This stabilization is shown by lower leakage currents, but the mechanism by which this is achieved is not understood.
Thus, it is desirable to increase the concentrations of both the depolarizer and the phosphate in electrolytes intended for high voltage service (200 V or higher) without sacrificing the voltage capabilities of these electrolytes. It is to this end that the present invention is addressed.